It was not clear if the gunman was trying to shoot Marois, whose party favors separation for the French-speaking province from Canada.
Montreal police Cmdr. Ian Lafreniere identified the gunman only as a 50-year-old man and said he opened fire in the back of the hall while Marois was giving her victory speech to hundreds of supporters at the Metropolis auditorium. She had just declared her firm conviction that Quebec needs to be a sovereign country before she was pulled off the stage.
"What's going on?" Marois told her security detail as they grabbed her arms and took her off the stage during the celebration of her party's victory in Tuesday's provincial election.
The gunman then fled outside where he set a small fire before he was captured, police said.
Police said they didn't know the gunman's motive. As the suspect was being dragged toward the police cruiser, he was heard shouting in French, "The English are waking up!"
Marois returned to the stage after the shooting and asked the crowd to peacefully disperse and then seemed to finish her speech. She left the hall amid a tight cordon of provincial police bodyguards.
The attack shocked Canadians who are not used to such violence at political events.
The suspect was a heavy-set man wearing a black ski or balaclava mask and a blue bathrobe over black clothes. Police didn't identify what weapons he had but camera footage showed a pistol and a rifle at the scene. Police said there is no reason to believe there are other suspects.
Police said a 45-year-old man was pronounced dead at the scene and a second man in his 30s was wounded. A third man was treated for shock. Police didn't identify the victims so it was not clear if any of them were party officials. The crowd was apparently unaware of what happened when Marois was whisked off the stage.
The separatist party won Tuesday's provincial election, but failed to win a majority of legislative seats. Though the Parti Quebecois wants the province to break away from Canada, its victory is unlikely to signal a new push for independence. Opinion polls show little appetite for a separatist referendum after previous ones had been rejected by voters in 1980 and 1995.
Marois herself has left much uncertainty about if and when a referendum would be held. But her party will push for more autonomy from the federal government.
The attack took place just after Marois began speaking in English - a rare occurrence in a speech at a partisan PQ event. She had promised English-speaking Quebecers that their rights would be protected, following an emotionally charged campaign that saw her party focus on language-and-identity issues. Earlier in the evening, people in the crowd booed when they heard outgoing Liberal Premier Jean Charest speak English in his concession speech, ending nearly 10 years in power.
More autonomy for Quebec is high on the agenda for the PQ, which has said it would seek a transfer of powers from the federal government in areas like employment insurance and immigration policy. If those measures are rejected, the party believes it would have a stronger case for independence.
Without a majority in the Quebec Assembly, however, the PQ will need to work with other parties to pass legislation, and the results will undermine efforts to quickly hold a referendum on separation.
The PQ had just under 31 percent of the vote and 54 seats in the provincial legislature, falling short of a majority in the assembly. The Liberals had about 31 percent and 50 seats.
A new party, Coalition Avenir Quebec, followed with 27 percent and 19 seats. The separatist Quebec Solidaire party won 2 seats.
A party needs to obtain 63 of the 125 seats to form a majority.
Before the shooting incident, Charest, who lost his own assembly seat, had congratulated Marois for becoming Quebec's first woman premier. He noted that she would be leading a minority government and said the results speak "to the fact that the future of Quebec lies within Canada." He did not indicate whether he intended to step down as Liberal leader after the defeat.
Earlier, Canadian Prime Minister Stephen Harper congratulated Marois on her victory but said he did not believe the results meant most Quebecers favor separation.
"We do not believe that Quebecers wish to revisit the old constitutional battles of the past," Harper said in a statement.
Harper's office had no immediate reaction to the shooting at the Parti Quebecois rally.
Although a number of candidates from the smaller parties are separatists, a minority government means "the more radical things in the party platform are going to be dead on arrival," said Bruce Hicks, a political science professor at Concordia University in Montreal.
Charest called the election more than a year before he had to, citing unrest in the streets due to this spring's student protests over tuition hikes. The most sustained student protests ever to take place in Canada began in February, resulting in about 2,500 arrests.
Marois, 63, was first elected to Quebec's National Assembly in 1981. She retired in 2006 but returned to become PQ leader a year later after her predecessor lost to Charest in an election that landed the PQ in third place. She in turn lost to Charest in 2008 but the 54-year-old Liberal leader seems to have lost his bet when he called early elections in August seeking a fourth mandate.
It's not the first time there has been political violence in Quebec related to tensions between the French and English. In the 1970s Canadian soldiers were deployed to the streets of Quebec because of a spate of terrorism by a group demanding independence from Canada. In 1970, the shadowy militant FLQ demanded "total independence" from Canada. Its members kidnapped and killed Quebec's labor minister and later abducted, then freed, a British diplomat.
The subsequent "October Crisis" was considered one of the darkest periods in modern Canadian history. Canadian troops patrolled the streets of Quebec and jailed alleged FLQ sympathizers, most of whom were later found innocent of having any FLQ ties.____
Gillies reported from Toronto